diff --git a/build/build_all b/build/build_all
index 02a4afa6..16cd95f1 100755
--- a/build/build_all
+++ b/build/build_all
@@ -291,11 +291,14 @@ fi
 ubuntu_clang_hack=0
 # If invoked by GitHub Actions CI...
 if [ -n "$RUNNER_OS" ]; then
-	case "$CC" in
-	clang*)
-		ubuntu_clang_hack=1
-		;;
-	esac
+	if [ "$os" = linux ]; then
+		# Assume that the OS being Linux means Ubuntu.
+		case "$CC" in
+		clang*)
+			ubuntu_clang_hack=1
+			;;
+		esac
+	fi
 fi
 
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